Applying Behaviors to Masks
You can apply behaviors to masks. When you apply a behavior to a mask, the mask is
animated like any other layer. However, because the mask only affects the transparency
in a layer, and not the position, rotation, or scale of the layer, the result is similar to that
of a “traveling matte,” where the mask moves in the layer it is applied to, hiding and
showing different areas of the image as it moves.
To apply a behavior to a mask
Do one of the following:
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Drag a behavior from the Library onto a mask in the Layers list or Timeline.
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Select a mask in the Layers list or Timeline, then choose a behavior from the Add Behavior
pop-up menu in the toolbar.
The behavior appears nested underneath the mask to which it’s applied.